Date: Tue, 25 Sep 2007 15:57:03 -0400 6 of 23 From: Losyuk Alexey Subject: Re: Occultation of 8.9-mag. star by possible rings of Pluto this week To: Planoccult list , David Dunham , "J. Lecacheux" , Bruno Sicardy Some remarks from a September 26th message from Jean Lecacheux (jlx) are added below. Dear David Dunham, J. Lecacheux, Bruno Sicardy, Thanks to you for valuable councils on observation of Pluto occultation by the star SAO 160793. Let me make some small remarks. 1. The rating of magnitude V=8.7 m of an occultated star SAO 160793 is taken from catalogue UCAC2 which is not photometric and contains very approximate values of star magnitudes. With using more exact (in the relation of photometry) The Tycho-2 Catalogue (Hog + 2000) we receive: V=8.945+/-0.033 m, B=10.351+/-0.058 m, a color parameter (B-V)=1.406 m. 2. Dave Herald wrote: > Attached are plots of this event, with a correction to the DE413 > position of -100mas and +60mas having been applied (as consistent > with recent occultations). With allowance for this corrections the greatest Pluto convergence with a target star for Majdanak observatory (MPC code 188, longitude 66deg 53' 47.0" E, latitude 38deg 40' 23.7" N, altitude 2575 m), will take place not at 14:48, but at 14:51 UT. Probable Charon occultation are expected at 15:02. 3. That is written on a web-page in Russian at http://www.referat-s.ru/mathematics/26405 . Astronomers of the Main astronomical observatory of the Russian academy of sciences (in Pulkovo) 60 years studied the periodic magnitude changes of Pluton - Charon system which have the period of 7.8 years. Look: Perov N. I. A method of localization of unknown minor bodies in multiple systems of major bodies / Proceedings of International Conference "AstroKazan-2001", September 24-29, 2001, Kazan State University: Publisher "DAC", 2001. P. 253-256. The helpful information also contains in paper by Rylkov V. P., Vityazev V. V., Dementieva A. A. Pluto: an analyses of photographic positions obtained with the Pulkovo normal astrograph in 1930-1992 / Astronomical Transactions. 1995. V. 6. P. 265-281. In this paper the model of particle movement within the limits of twice average limited problem of three bodies has been used, with allowance for Pluto compression. Authors have come out with the assumption about existence at distance of 2510-2520 kms from Pluto the hypothetical ring consisting of particles with characteristic radius 'Rr'. Let the radius of rings is equal to 3700 kms. Then the occultation by the rings will take place a 5.5 minutes before to the moment of maximal Pluto and star convergence (14:51 UT). The repeated occultation by the rings is necessary to expect in 5.5 minutes after it. 4. Unfortunately, the nearest comparison star S?? 160798 has the color strongly distinguished from the color of target star SAO 160793. Using The Tycho-2 Catalogue (Hog + 2000) it is possible to calculate: V=8.038+/-0.017 m, B=8.504+/-0.018 m, a color parameter (B-V)=0.466 m. It is bad. For photometry aspire to choose the stars having color close to color of observing star. At changing of height of a target star above horizon its observable magnitude will be (owing to atmospheric extinction) vary differently, than magnitude of a comparison star. The height of a star from 14 o'clock till 15 o'clock UT will change in Majdanak observatory from 32 up to 27 degrees. [jlx, 9/26, remarks:] You are right. As SAO 160793 is quite redder, the ratio *160793/*160798, close to 0.6 (?) high in the sky and with no filter, will increase with the air mass. Undoubtly a poor situation for quantitative measurements. However when recording fast events, of duration between 1 second (~ 10 km) and 3 minutes (2 000 km), the air mass hardly changes and this ratio may be considered as stable enough. The main advantage of SAO 160798 is to provide acceptable signal/noise ratio at standard video cadence with a small telescope. [end jlx 1st remark] However there is no any other star of comparison nearby, and field of view of CCD is a very small. [jlx, 9/26, remarks:] If your field is large enough (at least along its diagonal), there is the V= 10.3 reddish star TYC 6240 1041 at 6.7 arc min. NNW, which has the same colour as the target, although being three times fainter. [end jlx 2nd remark] 5. There is a question about the expected value of star magnitude falling at during the its occultation by the Pluto rings. As it known, Uranus rings were discovered by the results of star occultation observation in 1977. We can't find the direct reference on the value of magnitude falling for this event in the Internet. However Lupus (Vitaly Mechinsky) on the basis of abstract to article in "Nature" (Nature 267, 328 - 330 (26 May 1977); doi:10.1038/267328a0 "The rings of Uranus" J. L. ELLIOT, E. DUNHAM & D. MINK) and the data from http://nssdc.gsfc.nasa.gov/planetary/factsheet/uranringfact.html executed an indirect estimation of value of magnitude falling (look http://www.ka-dar.ru/forum/index.php/topic,702.msg7511.html#msg7511 ). He has shown, that magnitude falling for rings of Uranus was from 0.109 m (optical depth of ring t=0.1) up to 0.543 m (optical depth of ring t=0.5). Here t - the optical depth of media, the dimensionless value describing the attenuation of optical radiation in the media at the expense of joint action of light absorption and dispersion (but without effects of amplification of the radiation caused by multiple dispersion). We haven't access to "Nature" articles, and we can't to compare this results. So, maybe Vitaly is mistaken in his calculations. 6. Dear Bruno Sicardy, we watched closely for your page http://calys.obspm.fr/~sicardy/predic_occn_06_07/table.html during all first half-year 2007. Unfortunately, in the second half-year you have ceased to make calculations of stars occultations by Pluto and its satellites. Do you use ephemeris DE413 during the calculations of Pluto occultations? That writes Dave Herald: > Fairly soon I will be releasing a new version of Occult - in beta > form. A key improvement for events involving Pluto is that the > DE413 ephemeris can be downloaded for the predictions. DE13 was > generated a couple of years ago specifically for Pluto occultations. > The path location is _significantly_ different from the DE405 > ephemeris. Indeed, for Pluto occultations DE413 is effectively > mandatory. Besides this, Dave Herald considers, that it's necessary to note the correction for this ephemeris (as consistent with recent occultations). About it it is written above. Best regards, Losyuk Alexey